Refineries often use an available refinery fuel gas stream as part of the feedstock for an SMR, with the balance typically being natural gas. Some of these refinery fuel gas streams contain olefins such as ethylene, propylene, butenes, pantenes, etc. The concentration of the olefins in the fuel gas varies due to various changes in refinery operations. The concentration of the olefins in the feedstock can also increase if the natural gas feedstock is reduced relative to the refinery fuel gas stream.
Typically, these olefins are hydrogenated upstream of the SMR. The hydrogenation of olefins is an exothermic reaction. Due to the limitations on the hydrogenation catalyst, there is a limit on the allowable temperature rise within the hydrogenator. This limits the amount of olefins that may be allowable in the feed stream. This limit is typically between about 4% and about 6% by volume. For streams that contain higher concentrations of olefins, a recycle loop is usually used around the hydrogenator. This adds operational complexity, plus the capital and operating expenses relating to the addition of a recycle gas compressor and a recycle gas cooler.